The Secret Service has confirmed to USA TODAY that it is investigating a potential massive data breach involving shoppers' personal credit card information and Target retail stores.

"The Secret Service will confirm it is investigating the incident at Target," spokesman Brian Leary said in a telephone interview Wednesday night. "We don't have any further comment because it's an ongoing investigation."

The potential breach began around Black Friday, the day after Thanksgiving and the busiest shopping day of the year, according to the Krebs on Security website. The breach involves the theft of information stored on the magnetic stripe on the backs of cards used at nearly all of Target's stores around the country, Krebs reports.

Target is based in Minneapolis and has almost 1,800 stores in the United States and 124 in Canada, according to its website.

The discount retailer did not respond to a telephone message left Wednesday night at its corporate offices.

Krebs reports its sources initially thought the breach took place from just after Thanksgiving to Dec. 6, but now believe it took place until as far as Dec. 15.

The potential breach does not appear to involve online purchases, Krebs reports. It appears the type of data stolen would allow thieves to create counterfeit credit cards and, if pin numbers were intercepted, would also allow thieves to withdraw cash from ATM machines, according to Krebs.

MasterCard and Visa did not respond to e-mails or telephone messages left with their corporate offices.